Density Fluctuations in the Solar Wind Driven by Alfv\'en Wave Parametric Decay

TL;DR

This paper investigates whether Alfvén wave parametric decay can generate compressive fluctuations in the solar wind, finding it is overdamped at 1 AU but still correlates with observed density fluctuations.

Contribution

It provides a comparison of damping and growth rates to evaluate the role of parametric decay in producing solar wind compressive fluctuations.

Findings

Compressive waves are overdamped at 1 AU.

Presence of slow-mode density fluctuations correlates with Alfvén wave decay.

Parametric decay may influence solar wind turbulence despite damping.

Abstract

Measurements and simulations of inertial compressive turbulence in the solar wind are characterized by anti-correlated magnetic fluctuations parallel to the mean field and density structures. This signature has been interpreted as observational evidence for non-propagating pressure balanced structures (PBS), kinetic ion acoustic waves, as well as the MHD slow-mode. Given the high damping rates of parallel propagating compressive fluctuations, their ubiquity in satellite observations is surprising, and suggestive of a local driving process. One possible candidate for the generation of compressive fluctuations in the solar wind is Alfv\'en wave parametric instability. Here we test the parametric decay process as a source of compressive waves in the solar wind by comparing the collisionless damping rates of compressive fluctuations with the growth rates of the parametric decay instability daughter waves. Our results suggest that generation of compressive waves through parametric decay is overdamped at 1 AU, but that the presence of slow-mode like density fluctuations is correlated with the parametric decay of Alfv\'en waves.